This is going to freak out
caseydog , but I am a super organized person
and I have a Google spreadsheet where I keep track of all my spending and in the end of every month I total how much I spent and where. I even have separate columns for separate categories (ie housing, food, clothes, etc) making it easier to calculate each total.
This year I'm spending an average of 160€/month on food. That's my supermarket spending only, not counting eating out, and while I live alone my boyfriend spends 2/3 days every week at my house and since he eats different stuff for breakfast than me he buys his breakfast food but I buy the rest.
I love going to the supermarket, so every week I check each supermarket's flier so I know what's on sale. If any of the foods I buy regularly are cheap, I buy in bulk so I have enough stock to last me awhile (as long as it's non-perishable food or food that can be frozen). When I'm almost running out of that food I start looking for promotions again.
Something very important is to always compare prices in price per kg or price per liter, not just the price that's on the product. Sometimes products seem to be on sale, but when you look at price per kg/liter you realize there are other products that are not on sale that are cheaper.
The biggest issue is fresh produce, which I don't buy in bulk because I'm afraid of it spoiling. Luckily I recently found the tupperware ventsmart line, in my experience I can keep produce fresh for 3 weeks in these boxes. With onions, when they're on sale I buy in bulk, chop them and freeze them. It works really well for stews, which is most of the food I cook.